The present invention relates to a system for locating Fresnel reflections along an optical fiber, which system is designed in particular for measuring the lengths of optical fibers and for locating breaks along such fibers, in particular for applications to maintenance of communications cables.
In response to a light pulse sent along an optical fiber, two types of reflection are received at the input of the fiber:
reflections due to Rayleigh backscattering on the silica crystals; the maximum light power of these reflections is about 40 dB below the light power of the incident pulse; and
reflections due to ends (the front face and the rear face) of the fiber, and to the highly-reflective connectors situated along the fiber: these are Fresnel reflections; the maximum power of these reflections is about 14 dB below the light power of the incident wave.
In order to measure the length of an optical fiber (or to locate the position of a break, which comes to the same), it is necessary to detect the Fresnel reflection due to the rear face of the fiber under test.
Two methods may be used.
The first method uses the principle of reflectometry, based on Rayleigh backscattering. A light pulse is injected along an optical fiber to be tested, in general via a "lead-in fiber" whose exact length is known, and which is welded onto the front face of the fiber to be tested. The Fresnel reflection due to the front face of the fiber is of very high amplitude because the light constituting the reflection has not penetrated into the fiber, and consequently has undergone almost no attenuation. This leads to saturation of the detection system of the reflectometer. During the desaturation time, also referred to as the "dead period", no signal can be detected, and this is naturally detrimental to the correct operation of the apparatus. The object of using a lead-in fiber of sufficient length to enable desaturation is to mitigate this problem.
The signals reflected by the fiber then enable the reflectometry curve to be drawn. A reflectometry curve (see FIG. 1) gives the attenuation A in dB along the fiber as a function of the distance d (in km). Both of the above-mentioned types of reflection appear on curve 1. The flat portions 2, 3, and 4 correspond to the Rayleigh backscattering due to the silica crystals. The peaks 6 and 7 correspond to the Fresnel reflections due to the reflective connectors situated along the fiber or along the lead-in fiber. The peak 5 corresponds to the Fresnel reflection due to the front face of the fiber. The peak 8 corresponds to the Fresnel reflection due to the break in the fiber.
Reflectometry apparatuses are mainly used to determine attenuation characteristics of optical fibers. It is also possible to use reflectometry to locate points along an optical fiber that have given rise to Fresnel reflections. However, this use poses several problems.
Firstly, the coupled optical power injected into the fiber must be sufficient for the backscattered energy (which is at -40 dB) to be detected. The power injected is directly proportional to the area of the laser pulse. Since it is not possible to increase the power emitted by the laser without limit, it is necessary to work with wide pulses so as to obtain the desired optical power. However, the distance resolution of the apparatus is better when the pulse is narrow. The need for a minimum emission power threshold implies a poor distance resolution (of about 1 meter (1 m)) and a dead period equivalent to about 40 m.
Furthermore, once the reflectometry curve is displayed on the screen of the apparatus, in order to locate the point that gave rise to a Fresnel reflection, an identification cursor must be displaced to the start of the rise of the Fresnel peak under study. For reasons of clarity, the curve 1 shown in FIG. 1 is actually an expanded view of the curve obtained in practice, and what is really seen at each Fresnel peak is a portion of curve such as that shown in FIG. 2. It is therefore necessary to expand the horizontal scale at the Fresnel peak under study, in order to place the cursor exactly at the start of the rise of the peak. Once these two operations have been performed, the desired distance is obtained by means of a function integrated in the apparatus.
In order to determine the distance between two points that gave rise to Fresnel reflections, the measuring principle is similar and two cursors are placed at respective ones of the two peaks under study.
In this way, firstly, the distance measurements are not taken in real time, but rather after the curve has been displayed and after analog-to-digital conversion of the signal so that it can be processed by a microprocessor, and secondly, the remaining operations take a relatively long time and require qualified personnel. Therefore, it is inconvenient to use a reflectometer for quick on-site checks, particularly since it is an apparatus that is bulky, very costly, and designed, in fact, to supply characteristics (overall assessment) of the fiber that are not useful when maintaining communications cables, at which stage the essential requirement is to detect a break in a link (equivalent to an ohmmeter for copper wire). Therefore, the reflectometer is not the most suitable apparatus.
It is possible to use a second method for detecting Fresnel peaks, based on the echometry principle. In echometry, only the Fresnel peaks are taken into consideration, i.e. reflections are detected only if above a certain threshold. Therefore, it is no longer essential to inject optical power as high as in reflectometry. Consequently, narrow pulses can be used, thereby giving the apparatus a small dead period (equivalent to about 2 m) and contributing to obtaining good distance resolution (of about 25 cm).
Given that only signals of detectable amplitude are retained, it is not necessary to use a sophisticated amplifier (bandwidth, gain, etc.), and this reduces the cost of the apparatus compared with a reflectometer.
The principle of echometry is as follows: a narrow high-amplitude laser pulse is injected into the input of the fiber (e.g. made of silica), or of the lead-in fiber, and at the same time clock counting is triggered. The first Fresnel reflection that is detected stops the counting. Given the refractive index of the silica making up the fiber, and on the basis of the time indicated by the clock, a microprocessor calculates the distance between the front face of the fiber and the first point of the fiber that gives rise to a Fresnel reflection. This measurement is taken in real time (the time counted by the clock is automatically and simultaneously transformed into length by the apparatus, in about one second).
The problem with using such an apparatus can thus be seen: if the fiber under test does not include any reflective connectors, then the Fresnel peak detected does indeed correspond to the rear face of the fiber; but if the fiber has connectors before its rear face, then it is impossible to detect the corresponding Fresnel peaks because the clock will have been inhibited after the return of the first Fresnel reflection due, to the first connector.